Sheathed heater

Abstract: A sheathed heater comprising at least two core pieces (2a, 2b) of porcelain insulator separated to each other in the axial direction of said core piece (2a, 2b), at least two lead pins (3a, 3b) separated to each other and inserted into the core pieces (2a, 2b) axially, at least two internal heater coil segments (4a, 4b) wound around the at leaat two core pieces (2a, 2b), respectively, and a cylindrical metal sheath (1) into which the core pieces (2a, 2b) and the internal heater coil segments (4a, 4b) are inserted through a thermal and electrical insulation material (5), wherein adjacent ends of adjacent internal heater coil segments (4a, 4b) are electrically connected to the same lead pin, or lead pins (3a, 3b) of the same electric potential.

Description: This invention relates to a sheathed heater, and more particularly to an improvement of a sheathed heater for use at a higher temperature atmosphere and starts from the prior art mentioned in the description as ' conventional sheathed heaters' according to Figure 2 resp. to Figure 3. The conventional sheathed heater to be connected across a single-phase power source, as shown in Fig. 1, comprises a metal pipe 1, such as a stainless steel pipe, a core of porcelain insulator 2 made of magnesia or the like and arranged at the center of the metal pipe 1, a pair of lead pins 3a, 3b having base portions which are inserted into small 1 holes formed in and passing through said core of porcelain insulator 2, an internal heater coil 4 wound around said core of porcelain insulator 2 and a thermal and electrical insulation material 5 of magnesia or the like filled in a space formed between the metal pipe 1 and the core of porcelain insulator 2, both ends of said internal heater coil 4 being electrically connected to said pair of lead pins 3a, 3b, respectively. Such a sheathed heater is reduced in diameter by a subsequent swaging process. In such a conventional sheathed heater, however, if the lengtn thereof is too long the sheathed heater tends to bend when it is subjected to said swaging process. Accordingly, it was contemplated to divide axially the core of porcelain insulator 2 into two or more core pieces, for example, into three core pieces 2a∼2c, and a planar spacer 6 of magnesia or the like is inserted between adjacent core pieces, as shown in Fig. 2. Further, in the conventional sheathed heater, the heater coil 4 has a limitation in length due to the diameter of the heater coil wire and the value of voltage applied thereto. Accordingly, said heater coil 4 is also divided into three heater coil segments 4a ∼ 4c, each wound around a different core piece 2a∼ 2c, and the voltage of a single-phase power source is applied by lead pins 3a, 3b to both ends of each heater coil segment 4a, 4b or 4c. Specifically, as shown in Fig. 2, one end A1, A2 or A3 of the heater coil segment 4a, 4b or 4c is connected to one of said lead pins 3a, 3b at one end of each of the core pieces 2a, 2b or 2c, and the other end B1, B2 or B3 of each heater coil segment 4a, 4b or 4c is connected to the other of said lead pins 3a, 3b at the other end of each of the core pieces 2a, 2b or 2c. Fig. 3 shows a conventional sheathed heater to be connected across a three-phase power source. It is publicly known that in the sheathed heater the insulation ability of the thermal and electrical insulation material 5 and the spacer 6 of the magnesia or the like is lowered if the temperature is increased to more than about 800°C. However, in the sheathed heater as shown in Fig. 2 or Fig. 3, a gap formed between the adjacent core pieces becomes a nonheated portion, so that the gap should be made small, such as about 2 to 3 mm in order to equalize the temperature distribution. If this is the case, a voltage, such as 100 V is applied between said gap, so that an abnormal current flows through said gap to heat this portion, thereby causing the service life of the sheathed heater to be shortened. Various sheathed heaters comprising a rod shaped insulator core arranged within a metallic sheath and an internal heater element wound round the core and disclosed in EP-A-501788, US-A-4039995, US-A-4039996 and US-A-5247158. US-A-4 039 996 discloses an electric heating element comprising two dielectric cores within a metallic sheath. One core has three longitudinal openings and the other core has two longitudinal openings. A resistance coil is wound on each of the cores. Three metal pins fit into predetermined core openings, two through both cores and the third in one of the cores. One of the terminals of each resistance coil is electrically connected to one pin, the opposite terminal of one resistance coil is electrically connected to a second pin; and the opposite terminal of the other resistance coil is electrically connected to the third pin; whereby one pin serves as a common electrical connection for both resistance coils. An object of the present invention is to reduce an abnormal current flowing through a gap formed between adjacent core pieces. The invention provides a sheathed heater as set out in the appended claims. Fig. 1 is a sectional view of a conventional sheathed heater to be connected across a single-phase electric power source; Fig. 2 is a sectional view of an another conventional sheathed heater to be connected across a single-phase electric power source; Fig. 3 is a sectional view of an other conventional sheathed heater to be connected across a three-phase electric power source: Fig. 4 is a sectional view of a sheathed heater to be connected across a single-phase electric power source according to an embodiment of the present invention; Fig. 5 is a sectional view of a sheathed heater to be connected across a three-phase electric power source according to the other embodiment of the present invention; and Fig. 6 is a sectional view taken along a line 6-6 in Fig. 5. Now, an embodiment of this invention will be described by referring to Figs. 4 to 6 In a sheathed heater to be connected across a single-phase electric power source according to an embodiment of the present invention, as shown in Fig. 4, one end A1 of a first internal heating coil segment 4a wound around a first core piece 2a is electrically connected to one lead pin 3a at one end of said first core piece 2a, whereas the other end B1 of said first internal heater coil segment 4a is electrically connected to the other lead pin 3b at the other end of said first core piece 2a. An end A2 of a second internal heating coil segment 4b wound around a second core piece 2b adjacent to said first core piece 2a is electrically connected to said other lead pin 3b at one end of said second core piece 2b, whereas, the other end B2 of said second internal heating coil element 4b is electrically connected to said one lead pin 3a at the other end of said second core piece 2b. Similarly, an end of A3 of a third internal heating coil segment 4c wound around a third core piece 2c adjacent to said second core piece 2b is electrically connected to said one lead pin 3a at one end of said third core piece 2c, whereas, the other end B3 of said third internal heating coil segment 4c is electrically connected to said other lead pin 3b at the other end of said third core piece 2c. Figs. 5 and 6 show a sheathed heater to be connected across a three-phase electric power source according to the other embodiment of the present invention. In this embodiment, three small holes separated in the circumferential direction from one another are formed in and passing through each of first to three core pieces 2a to 2c of porcelain insulator, and three lead pins 3a to 3c are inserted into said first to three small holes, respectively. As the first embodiment, one end A1 of a first internal heating coil segment 4a wound around said first core piece 2a is electrically connected to said first lead pin 3a at one end of said first core piece 2a, the other end B1 of said first internal heater coil segment 4a is electrically connected to said second lead pin 3b at the other end of said first core piece 2a, an end A2 of a second internal heating coil segment 4b wound around said second core piece 2b adjacent to said first core piece 2a is electrically connected to said second lead pin 3b at one end of said second core piece 2b, and the other end B2 of said second internal heating coil element 4b is electrically connected to said third lead pin 3c at the other end of said second core piece 2b. Similarly, an end of A3 of a third, internal heating coil segment 4c wound around said third core piece 2c adjacent to said second core piece 2b is electrically connected to said third lead pin 3c at one end of said third core piece 2c, whereas, the other end B3 of said third internal heating coil segment 4c is electrically connected to said first lead pin 3a at the other end of said third core piece 2c. Said three lead pins are electrically connected across a three-phase electric power source, respectively. As stated above, in the present invention, no voltage difference is formed between the adjacent ends of the internal heating coil segments adjacent to each other, such as B1-A2, B2-A3, because these are electrically connected to the same lead pin. Accordingly, the abnormal current is prevented from flowing between said adjacent lead pins, so that the service life of the sheathed heater can be prolonged. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A sheathed heater comprising a cylindrical metal sheath (1), a plurality of rod shaped core pieces (2a-2c) of porcelain insulator arranged in separated end to end adjacent relationship in said cylindrical metal sheath (1), a thermal and electrical insulation material (5) filled in a space formed between said metal sheath (1) and core pieces (2a-2c), a plurality of internal heater coil segments (4a,4b,4c) each wound around a different one of said core pieces (2a,2b,2c) and having opposed terminals located at . opposite ends of said core piece (2a,2b or 2c), and two lead pins (3a,3b) separated from each other and inserted into said core pieces (2a-2c), characterized in that : adjacent terminals (B1,A2;B2,A3) of adjacent internal heater coil segments (4a-4c) are connected to the same one of said two lead pins (3a,3b), and in that the non adjacent terminals of said adjacent internal leater coil segments (4a ∼ 4c) are connected to the other one of said two lead pins (3a, 3b). A sheathed heater according to Claim 1 characterised in that : the heater comprises three coil segments (4a-c); A sheathed heater according to Claim 1 or Claim 2, further comprising spacers (6) of thermal electrical and insulation material each inserted between adjacent core pieces (2a-2c) and wherein said two lead pins (3a,3b) are each inserted into said core pieces (2a-2c) and through said spacers (6). A sheathed heater comprising a cylindrical metal sheath (1), a plurality of rod shaped core pieces (2a-2c) of porcelain insulator arranged in separated end to end adjacent relationship in said cylindrical metal sheath (1), a thermal and electrical insulation material (5) filled in a space formed between said metal sheath (1) and core pieces (2a-2c), a plurality of internal heater coil segments (4a,4b,4c) each wound around a different one of said core pieces (2a,2b,2c) and having opposed terminals located at opposite ends of said core piece (2a,2b or 2c), and three lead pins (3a-3c) separated from each other and inserted into said core pieces (2a-2c), characterized in that : adjacent terminals (B1,A2;B2,A3) of adjacent internal heater coil segments (4a-4c) are connected to the same one of said three lead pins (3a-3c), and in that the opposed terminals (A1,B1;A2,B2;A3,B3) of each of said coil segments (4a-4c) are connected to a different one of said three lead pins (3a-3c) such that between each pair of lead pins (3a-3c) at least one of coil segments (4a - 4c) is connected. A sheathed heater according to Claim 4, further comprising spacers (6) of thermal electrical and insulation material each inserted between adjacent core pieces (2a-2c) and wherein said three lead pins (3a,3b,3c) are each inserted into said core pieces (2a-2c) and through said pacers (6). The sheathed heater according to Claim 4 or Claim 5, wherein said three lead pins (3a-3c) are separated from one another in the circumferential direction.

Inventors: NAGANO SHIGETOSHI, NAGANO, SHIGETOSHI
Applicants: SAKAGUCHI DENNETSU KABUSHIKI K, SAKAGUCHI DENNETSU KABUSHIKI KAISHA
Application year: 1994
Main classification: H05B3
IPC codes: H05B3